The invention relates to a method as generally defined hereinafter and also to a fuel injection system. A method and a fuel injection apparatus are already known in which when the engine is started, fuel is injected into the air intake tube of the engine via a supplementary, electromagnetically actuated injection valve. In order to enrich the fuel-air mixture at predetermined operating conditions of the engine, it is known to increase the pressure drop at the metering valves by reducing a control pressure by means of an electromagnetically actuatable control pressure valve.